The traditional technology for designing and manufacturing color image sensing devices has had a few technical difficulties and limitations. More particularly, color image sensors in the prior art have problems of low sensitivity, low pixel resolution, and color aliasing. Generally, an image sensor today can only sense either black and white images or color images. At present, there are only few methods in practice for making color image sensors capable of producing color images from single sensing pixel arrays. The most common method of making color image sensors is to coat a color filter pattern on sensing pixel arrays. Two color filter patterns are mainly used in a traditional color image sensor chip. FIG. 1 illustrates a color filter pattern, namely a CYMG (M for pink or Magenta) pattern (also called composite color filter pattern), which comprises cyan, yellow, magenta and green colors. FIGS. 2(a), 2(b), 2(c) and 2(d) and FIGS. 3(a) and 3(b) illustrate some primary color (RGB) filter patterns ordered as a Bayer pattern or a honeycomb pattern, respectively. Both of these two patterns sense red, green and blue colors.
In the color image sensors of CYMG pattern, the pixel array comprises many macro-pixels. Each macro-pixel comprises four pixels, each being coated by C, Y, M or G color filter, respectively. However, it is primary colors (namely RGB) pattern not CYMG pattern that is used in the display industry, and thus it is necessary to transform a color matrix for C, Y, M or G color to a matrix for RGB so as to convert CYMG pattern into RGB pattern. Furthermore, because each pixel senses only one color (either cyan, or yellow, or magenta, or green), to sense RGB colors by each pixel, interpolation technique is needed to interpolate the missed colors from the adjacent pixels. In Bayer pattern (U.S. Pat. No. 3,971,065), the sensing pixel array comprises many macro-pixels, each comprising four pixels coated with only RGB colors. Bayer pattern further requires that in every macro-pixel, two elements at one of the diagonals must sense green or a color corresponding to luminance of the image, whereas the other two colors sensed are red and blue, or colors corresponding to two other different spectra of visible light. Similarly, because each pixel senses only one type of color (red, or green, or blue), to sense the other two missed colors at the pixels, interpolation is needed to interpolate the missed colors from the adjacent pixels. Bayer pattern has four different orderings, each representing a certain arrangement of the RGB position. In a honeycomb pattern as shown in FIG. 3, a macro-pixel comprises only three pixels coated by RGB colors and arranged in a honeycomb form. In the honeycomb pattern, pixels sensing RGB colors are arranged uniformly and symmetrically; and exchanging the position of two pixels still yields a honeycomb pattern.
As described above, there are three common issues in implementing the color filters formed by a composite (CYMG) pattern, Bayer pattern or honeycomb pattern: firstly reducing light-sensing sensitivity due to the existence of the color filters (compared with the monochrome sensors); secondly, reducing effective spatial definition (or resolution) due to color interpolation, which in turn causes the third one, color aliasing. Normally, the color aliasing may be solved by using low-pass filters. However, low-pass filters will reduce the image definition, thereby worsening the second issue.
To avoid the reduction of the light sensitivity caused by the color filters, U.S. Pat. No. 6,137,100 discloses a method for balancing the sensing response of RGB pixels, which makes use of the characteristic of photodiodes that have different sensitivities for different colors. Specifically, a photodiode is more sensitive to green, secondly red, and then blue. Therefore, areas sensitive to blue are made biggest, then to red and smallest to green. The improvement on color sensitivity with this method is still limited. Moreover this method just emphasizes the RGB color pattern.
Recently, Kodak Company launched a method that combines white and RGB colors, namely adding a white sensing pixel to the RGB pixel array to increase the sensitivity. As shown in FIGS. 4(a), 4(b) and 4(c), as the white pixel absorbs several times more light energy than primary color (red, green or blue) pixels or the complementary color (cyan, yellow or magenta) pixels, the WRGB (white and RGB colors) method is certainly 2-3 times more sensitive than the sensors used in the traditional Bayer pattern. However, this method brings new problems. Firstly, the color reconstruction is more complicated. Secondly, as a result of changing three colors to four colors, the spatial resolution is reduced compared to Bayer pattern. Lastly, because the sensitivity of the white color is 6-10 times more than the other three RGB colors, the signal strengths of different colors are strongly mismatched, which limits the advantage of the high sensitivity of the white color, as the color with the lowest sensitivity determines the quality of an image.
In order to avoid color interpolation, Foveon Company of USA invented a new color sensing technology that uses three layers of sensing pixels, as shown in FIG. 5. A three-layer color image sensor, called “X3 image sensor”, has three layers of sensing arrays, each being sensitive to one light spectrum of the RGB colors, respectively. The X3 image sensor can solve the problem of color interpolation, but it brings out new problems due to the sensitivity differences of different sensing layers. The sensing sensitivity of a lower layer is usually lower than an upper layer of the three layers. Thus, the total effective sensitivity is reduced. In addition, the cost and complexity will be increased due to the manufacturing of the three layers. Furthermore, three times more data to be transmitted and processed significantly increase the system cost and power consumption of the X3 image sensor.
Color image sensors generally respond to the continuous spectrum of RGB color. There are also monochrome image sensors that are sensitive to the entire visible spectrum, or the infrared spectrum, or both of them. The sensitivity of this kind of monochrome sensor is generally 10 times more than the sensitivity of the traditional Bayer pattern image sensors (under the same physical condition of production), but such a sensor cannot produce color.
As mentioned previously, although many improvements on color sensing devices have been made in the prior art, each just has been improved in one or more aspects while lowering the performance of others. Single layer image sensors do not maximize use of the light energy while reducing the spatial resolution. Three-layer sensors fail to use the white color and the complementary colors, reducing sensitivity. In addition, manufacturing the three-layer sensors is overly complicated.
Hence, it is still necessary to improve the prior art to find out a sensing device and a manufacturing method thereof, which may combine the advantages of monochrome image sensors and color image sensors to solve the technical issues in the art as stated above.